A water-barometer could be made, but it would be an unwieldy thing, for the tube must be over 34 feet long. Besides, it would not answer in very cold weather, as the water would freeze. So short a column of the heavy fluid, mercury, balances the weight of the atmosphere that a barometer made with this is of very convenient size; and then there is no danger of the mercury's freezing, except in the extreme cold of the Arctic regions. 170. =Barometer a Measurer of Heights.=--The atmosphere, as stated in § 152, diminishes regularly in density as we go upward. The rate of this diminution has been accurately ascertained, and therefore we can estimate heights by the amount of pressure on the mercury in the barometer. At a height of 500 feet the barometer will be half an inch lower than in the valley below. At the summit of Mont Blanc it stands but half as high as at its foot, indicating a height of 15,000 feet. Du Luc, in his famous balloon ascension from Paris, saw the barometer at one time standing at about twelve inches, showing an elevation of 21,000 feet. 171. =Relation of the Air's Pressure to the Boiling Point.=--Water heated to 212 degrees of Fahrenheit boils, that is, it becomes vapor. Now if water be heated on the summit of a high mountain it boils before it arrives at this degree of temperature. On the top of Mont Blanc it boils at 180 degrees, that is, 32 degrees below the boiling point of water at the foot of the mountain. This is because the pressure of the air acts in opposition to the change of water in to vapor, and the less the pressure is the less heat will be required to vaporize the water. We may illustrate this influence of the pressure of air upon boiling by the following experiment. Let a cup of ether (which boils at 98 degrees) be placed under the receiver of an air-pump. On rarefying the air by the pump the ether will boil. The general effect of pressure upon boiling may be prettily illustrated by another experiment. Boil some water in a thin flask over a spirit-lamp. Blow out the lamp, and, corking the flask tightly, let the boiling cease. If, now, you pour some cold water over the flask the boiling will commence again with considerable force. Why? Because you condense the steam which is over the water by the application of cold, and thus take off the pressure. Then, again, if, while the water is boiling, you pour hot water over the flask, the boiling ceases, because the heat favors the accumulation of steam, and therefore renews the pressure on the surface of the water. You can see from what has been stated that most liquids have the liquid form because of the pressure of the atmosphere upon them. If there were no atmosphere, ether, alcohol, the volatile oils, and even water, would fly off in vapor; and the earth would be enveloped in a vaporous robe, for the particles of the vapors would be held to the earth by attraction, just as the particles of the air are now, § 151. [Illustration: Fig. 117.]